What are the normal measures taken for yelling at a fellow employee and throwing paper the table?

My mom was working for Daktronics for about 4 years now and she's recently been fired from data entry position.

She's the ONLY minority working in her department, and she has been harassed several times by her coworkers. Harassment includes: assigning her their own work, treating her as beneath her, verbally gossiping, and distracting her from work for 4 years. She never provoked or did their work and has addressed this problem multiple times with her supervisor. The problem has been addressed by her previous boss, who started assigning work to individuals.

My mom has been professionally complemented on her efficiency at work and correctness. All she does is keep to herself.

Recently the three other employees put a pile of their assigned work on my mom's desk to do. My mom went back to their desk and yelled "You are not my supervisor," and slammed the pile of work on their desk.

Now this new boss, sent my mom home and terminated her position. This new boss from Daktronics was fully aware of the situation before it escalated, not only did she fail to address it, she has racially discriminated against my mother (who speaks English with an accent). When my mother was hired, there were three panelist. One of which was this recent employer, who asked my mom are you capable of typing with two fingers? and a series of questions that were unprofessional and doubted my mother's credentials while the other two asked basic questions.

My mom was fired for retaliation. I need to know, under what has happened were they legally right to call her actions retaliation?

We called Human Resources, and 1/2 hour into the conversation, the following question was asked:
"is there another person to which we can address this concern?" the word discrimination was never brought up in conversation.

The lady just was silent, did not hang up or respond, just was silent.

What's the smartest move to make in this situation.

An answer would be greatly appreciated. Please help.